constants: Refactoring.
constants: Add pkg docs.
constants: Delete MC Protocol version.

- Protocol version is already on `proto/minecraft.Version`.
proto/minecraft: Don't read PacketId on ReadFrom.
proto/minecraft: Remove unnecessary comments.
proto/minecraft: Fix 0xFC + 0xFD array length.

- Length of slices is stored inside them, this let's us use WriteTo
correctly (there's no need to fill those 2 extra fields).